The present invention relates to semiconductor devices that are MIS capacitors and methods for producing the same, in particular, semiconductor devices that are MIS capacitors that can be produced in a high yield.
Conventionally, as shown in FIGS. 6(a) and 6(b), a semiconductor device 100 that is a MOS capacitor includes an isolating region 114 on a silicon substrate 112, and an active region 116 is formed in an upper portion of the silicon substrate 112 enclosed by the isolating region 114. The dashed line of FIG. 6(a) is a boundary line between the isolating region 114 and the active region 116 (a capacitive insulating film 118). The capacitance insulating film 118 is formed on the active region 116. A rectangular upper electrode 121 is provided on the capacitance insulating film 118 so as to be spaced away from the isolating region 114. On the separating portion 114, a rectangular ring electrode pad 124 is provided so as to enclose the active region 116. Each side of the electrode pad 124 and each side of the upper electrode 121 are connected to each other via a lead conductive film 122. An interlayer insulating film 126 is formed above the substrate, and multiple contact holes 128 (a diameter of 0.28 μm) penetrating the interlayer insulating film 126 to reach the electrode pad 124 are opened. Plugs obtained by filling a conductive film are formed in the contact holes 128, and the electric potential of the upper electrode is controlled via these plugs.
However, the semiconductor device 100 that is a conventional MOS capacitor has the problem that the breakdown of the capacitance insulating film 118 occurs when plasma etching is performed with respect to the interlayer insulating film 126 to form the contact holes. When the portion of the capacitance insulating film 118 in which breakdown had occurred was investigated by emission spectrometry, it was found that all the breakdown of the capacitance insulating film 118 occurred in a boundary portion Rcr that is in contact with the isolating region 114 below the lead conductive films 122. Then, the inventors of the present invention concluded with regard to the mechanism causing this problem as follows.
FIGS. 7(a) and 7(b) are views illustrating the mechanism that causes breakdown of the capacitance insulating film.
As shown in FIG. 7(a), the interlayer film 126 is etched primarily by positive ions extracted from a plasma, and when the contact hole 128 penetrates the interlayer film 126, the surface of the electrode pad 124 is exposed. At this time, the electrode pad 124, the upper electrode 121, and the lead conductive film 122 are positively charged with the incident positive ions. In general, the positive charges are electrically neutralized by the negative ions and electrons that are alternately incident to the contact holes by using a high frequency plasma. However, as shown in FIG. 7(b), the surface of the substrate is negatively charged with electrons. Therefore, when the contact hole 128 having a high aspect ratio is formed, the electrons become susceptible to the influence of the negatively charged surface of the substrate (surface of the resist film), so that the electrons become difficult to be incident to the contact hole 128.
Furthermore, the positive ions in plasma are incident to the substrate while being accelerated by an electric field in an ion sheath generated in the plasma. The generation of the ion sheath results from the difference in the mobility between positive ions and electrons, that is, the fact that positive ions are difficult to move because the mass of a positive ion is larger than that of an electron. As a result, a negative electric potential called “self bias” occurs on the surface of the substrate. The positive ions collide with the surface of the substrate while bearing an energy of the potential in the plasma and the self bias. On the other hand, the electrons are difficult to be incident into the contact hole because the ion sheath acts as a decelerating electric field.
As described above, when the positive charges are accumulated in the electrode pad 124, the upper electrode 121, and the lead conductive film 122, an electric filed should be concentrated in the boundary portion Rcr of the capacitance insulating film 118 that is in contact with the isolating regions 114 below the bent lead conductive film 122. Then, when a voltage exceeding the breakdown voltage of the capacitance insulating film 118 is applied between the upper surface and the lower surface of the boundary portion Rcr of the capacitance insulating film 118 that is in contact with the isolating region 114, the breakdown of the capacitance insulating film 118 occurs in the boundary portion Rcr. The capacitance insulating film 118 is formed by thermal oxidation or CVD after the isolating region 114 is formed. There are a large amount of impurities and defects in a portion of the isolating region 114 bordering the active region. Therefore, when the capacitance insulating film 118 is formed by thermal oxidation, the capacitance insulating film 118 in the boundary portion Rcr that is in contact with the isolating region 114 is thin, so that the breakdown voltage in the boundary portion Rcr of the capacitance insulating film 118 that is in contact with the isolating region 114 is reduced. Furthermore, when the capacitance insulating film 118 is formed by CVD, the film quality of the capacitance insulating film 118 in the boundary portion Rcr that is in contact with the isolating region 114 is deteriorated by impurities, so that the breakdown voltage in the boundary portion Rcr of the capacitance insulating film 118 that is in contact with the isolating region 114 is reduced. Thus, in either case, breakdown of the capacitance insulating film is likely to occur.
In order to solve the above-described problems, it is believed to be effective to adjust the antenna ratio to be a certain value or more. The antenna ratio refers to the ratio of the total sum (S) of the exposed areas of the electrode pad 124 in the contact holes 128 to the area of the upper electrode 121. The breakdown ratio of the capacitance insulating film of the semiconductor device 100 that is a MOS capacitor when changing the total sum (S) of the exposed areas of the electrode pads 124 in the contact holes 128 was measured. FIG. 8 shows the measurement results thereof.
FIG. 8 is a graph showing the dependence of the capacitance insulating film breakdown ratio on the ratio of the total sum (S) of the exposed areas of the electrode pad in the contact holes to the area of the upper electrode 121. In FIG. 8, the horizontal axis shows the ratio (antenna ratio) of the total sum (S) of the exposed areas of the electrode pad in the contact holes to the area of the upper electrode, and the vertical axis shows the capacitance insulating film breakdown ratio. These results indicate that when the antenna ratio is relatively small, the breakdown of the capacitance insulating film 118 is more likely to occur. However, when the antenna ratio is too small or too large, the breakdown of the capacitance insulating film does not occur, and there is no correlation between the antenna ratio and the breakdown of the capacitance insulating film. That is to say, it cannot be said that it is effective to adjust the antenna ratio to be a certain value or more in order to suppress or prevent the breakdown of the capacitance insulating film in the semiconductor that is a MOS capacitor.